centos man page for mq_notify

MQ_NOTIFY(3)						     Linux Programmer's Manual						      MQ_NOTIFY(3)

NAME
       mq_notify - register for notification when a message is available

SYNOPSIS
       #include <mqueue.h>

       int mq_notify(mqd_t mqdes, const struct sigevent *sevp);

       Link with -lrt.

DESCRIPTION
       mq_notify() allows the calling process to register or unregister for delivery of an asynchronous notification when a new message arrives on
       the empty message queue referred to by the descriptor mqdes.

       The sevp argument is a pointer to a sigevent structure.	For the definition and general details of this structure, see sigevent(7).

       If sevp is a non-NULL pointer, then mq_notify() registers the calling process to receive message notification.  The sigev_notify  field	of
       the sigevent structure to which sevp points specifies how notification is to be performed.  This field has one of the following values:

       SIGEV_NONE
	      A "null" notification: the calling process is registered as the target for notification, but when a message arrives, no notification
	      is sent.

       SIGEV_SIGNAL
	      Notify the process by sending the signal specified in sigev_signo.  See sigevent(7) for general details.	The si_code field  of  the
	      siginfo_t  structure  will be set to SI_MESGQ.  In addition, si_pid will be set to the PID of the process that sent the message, and
	      si_uid will be set to the real user ID of the sending process.

       SIGEV_THREAD
	      Upon message delivery, invoke sigev_notify_function as if it were the start function of a new thread.  See sigevent(7) for details.

       Only one process can be registered to receive notification from a message queue.

       If sevp is NULL, and the calling process is currently registered to receive notifications for this message queue, then the registration	is
       removed; another process can then register to receive a message notification for this queue.

       Message	notification  occurs  only  when a new message arrives and the queue was previously empty.  If the queue was not empty at the time
       mq_notify() was called, then a notification will occur only after the queue is emptied and a new message arrives.

       If another process or thread is waiting to read a message from an empty queue using mq_receive(3), then any message notification  registra-
       tion is ignored: the message is delivered to the process or thread calling mq_receive(3), and the message notification registration remains
       in effect.

       Notification occurs once: after a notification is delivered, the notification registration is removed, and another process can register for
       message notification.  If the notified process wishes to receive the next notification, it can use mq_notify() to request a further notifi-
       cation.	This should be done before emptying all unread messages from the queue.  (Placing the queue in nonblocking mode is useful for emp-
       tying the queue of messages without blocking once it is empty.)

RETURN VALUE
       On success mq_notify() returns 0; on error, -1 is returned, with errno set to indicate the error.

ERRORS
       EBADF  The descriptor specified in mqdes is invalid.

       EBUSY  Another process has already registered to receive notification for this message queue.

       EINVAL sevp->sigev_notify  is  not  one of the permitted values; or sevp->sigev_notify is SIGEV_SIGNAL and sevp->sigev_signo is not a valid
	      signal number.

       ENOMEM Insufficient memory.

       POSIX.1-2008 says that an implementation may generate an EINVAL error if sevp is NULL, and  the	caller	is  not  currently  registered	to
       receive notifications for the queue mqdes.

CONFORMING TO
       POSIX.1-2001.

EXAMPLE
       The following program registers a notification request for the message queue named in its command-line argument.  Notification is performed
       by creating a thread.  The thread executes a function which reads one message from the queue and then terminates the process.

       #include <pthread.h>
       #include <mqueue.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define handle_error(msg) 
	   do { perror(msg); exit(EXIT_FAILURE); } while (0)

       static void		       /* Thread start function */
       tfunc(union sigval sv)
       {
	   struct mq_attr attr;
	   ssize_t nr;
	   void *buf;
	   mqd_t mqdes = *((mqd_t *) sv.sival_ptr);

	   /* Determine max. msg size; allocate buffer to receive msg */

	   if (mq_getattr(mqdes, &attr) == -1)
	       handle_error("mq_getattr");
	   buf = malloc(attr.mq_msgsize);
	   if (buf == NULL)
	       handle_error("malloc");

	   nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
	   if (nr == -1)
	       handle_error("mq_receive");

	   printf("Read %ld bytes from MQ
", (long) nr);
	   free(buf);
	   exit(EXIT_SUCCESS);	       /* Terminate the process */
       }

       int
       main(int argc, char *argv[])
       {
	   mqd_t mqdes;
	   struct sigevent sev;

	   if (argc != 2) {
	    fprintf(stderr, "Usage: %s <mq-name>
", argv[0]);
	    exit(EXIT_FAILURE);
	   }

	   mqdes = mq_open(argv[1], O_RDONLY);
	   if (mqdes == (mqd_t) -1)
	       handle_error("mq_open");

	   sev.sigev_notify = SIGEV_THREAD;
	   sev.sigev_notify_function = tfunc;
	   sev.sigev_notify_attributes = NULL;
	   sev.sigev_value.sival_ptr = &mqdes;	 /* Arg. to thread func. */
	   if (mq_notify(mqdes, &sev) == -1)
	       handle_error("mq_notify");

	   pause();    /* Process will be terminated by thread function */
       }

SEE ALSO
       mq_close(3), mq_getattr(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7), sigevent(7)

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2010-10-04							      MQ_NOTIFY(3)